I have a Canon 700d and a 430EXIII flash. Recently I have been experimenting with off-camera flash. I was really surprised to see e-ttl ii available even when using off-camera flash (via the pop-up flash)

Now, I understand (or at least I think I understand) what TTL does (or whatever Canon calls it) when the flash is mounted on the camera.

  1. Flash sends a preflash and sees what reaches the lens
  2. A proper exposure is calculated
  3. Flash adjusts its power to reach that proper exposure
  4. Actual flash is shot along with shutter

However if the flash is off-camera, step 3 is no longer straight-forward. I may have placed the flash on my scene in a position where it is impossible to obtain a proper exposure no matter the power of the flash.

As a quick example, let's say that I want to use the flash as a back-light for a portrait.

During the pre-flash nothing will really change on what is shown through the lens apart from some small highlights on the hair of the person.

So if the flash assumes that it needs a lot of power to obtain the proper exposure (because let's say that the face of my model is really underexposed) its batteries will be wasted as it goes to full power since 99% of its light never changes the picture.

Is my assumption correct?

Or does TTL behave differently when on-camera and off-camera?

  • Of course you can think of situations where TTL fails, but it doesn't follow that TTL isn't useful. – Caleb Jun 25 '16 at 5:05

The same is true when the flash is on the camera, facing away from the subject into empty space with nothing to reflect it back to the subject.

The point of TTL is to adjust flash power automatically under the assumption that it has an influence on the scene.

If the flash is on camera or not is not too relevant. In event photography or photojournalism for example, you can sometimes see photographers holding the flash in one hand and the camera in the other. This gives more flexibility in positioning the flash. In run and gun scenarios as those mentioned, an automatic flash that helps getting the shot is more important than achieving perfect exposure.

its batteries will be wasted as it goes to full power since 99% of its light never changes the picture.

I often bounce on-camera flash into high ceilings or walls, because I want to get soft light. This also leads to high power flashes. I rarely have battery problems.

Your thought is right: all the light that does not eventually reach the sensor is wasted, but there's no simple way to increase that efficiency.

Some systems are coupled with the autofocus to determine flash settings based on subject distance, but this works only on-camera and only without tilting or rotating the head of the speedlight.

In summary: while not achieving perfect exposure for the reasons you mentioned, TTL is still useful either on or off-camera to get a good exposure automatically.


Don't forget that when using E-TTL you can also apply different power ratios and flash exposure compensation to each group independently. In your example of the hair light you can either change the power ratio of the key light to the hair light. Or if you are only using a single flash you can dial it up or down using FEC. And by selecting the Av exposure mode you can tell your Canon EOS camera that you want the ambient light to be the main light source.

As others have already said, E-TTL is most useful in situations, such as event photography, when the subjects' positions and the ambient lighting is changing rapidly. You don't have to constantly change aperture or flash power every time the subject gets closer or further from your lights. Even when you have lights mounted remotely on stands you can adjust the ratio so that when you are in one part of the room E-TTL gives more power to the flash more behind you (key) and less to the one more in front of you (kicker). Then when you move across the room you only need to adjust the ratio to flip the entire key/kicker setup. It's a lot quicker changing one setting (especially if that setting is on your customizable "quick menu" list) than to manually change the power of each group individually.

However if the flash is off-camera, step 3 is no longer straight-forward. I may have placed the flash on my scene in a position where it is impossible to obtain a proper exposure no matter the power of the flash.

The same is true when the flash is on camera. Imagine you are too far from your subject for proper exposure even when on full power and pointed directly at the subject. Or suppose you are bouncing the flash off a wall or ceiling that absorbs too much of the light.

Using E-TTL in Av exposure mode is specifically tailored to attempt to expose properly using the ambient light as the main light source and then only using the flash as fill or accent light. If the ambient light falls below a certain exposure level (based on the minimum Tv or maximum ISO you have told the camera to use) the the camera kicks over to using the flash as the primary light source. This is true not only in the Canon EOS system, but also in many other camera makers' systems.

Or does TTL behave differently when on-camera and off-camera?

It depends. On or off camera, if the flash head is tilted up or swiveled to the left or right the E-TTL system will take that into account and won't use the focus distance reported by the lens in the calculation of how much power to use. If off camera using optical wireless control (such as a camera mounted or pop up flash) the camera won't use lens focus distance either. If you're using wireless radio triggers, though, the camera may think the flash is camera mounted and try to use lens reported focus distance if the flash is set with the head pointing level and straight ahead.


The basic answer is really that TTL flash isn't all that important in many situations, and that most of the situations where off-camera flash might be used fall into that camp.

If you're in a place where the light doesn't change much, any exposure automation is not a big deal, let alone flash automation. It's handy for the first exposure of the session, and after that, you can just leave it alone. With flash, you can take a test shot or two, and there you go.

If the flash is on-camera and you're pointing it directly at your subject, the needed power may vary as you're moving around. But, that's generally going to produce inferior results, so you don't want to do that anyway. If, instead, you're bouncing off of the ceiling, your position won't matter as much. And, following that thought, it also doesn't matter much if the flash is on the camera or in a corner somewhere. Sure, having TTL makes it so that you don't have to think to adjust the flash power to compensate if you change aperture, but with any remote manual control that's pretty easy.

If you're moving around a lot and in a big area with a lot of different lighting conditions and subject distances, TTL flash is nice. As Michel Clark explains, this can apply to multiple flashes when doing event photography in large spaces.

On the other hand, in a smaller scale — studio, or ad-hock studio-like lighting on location, where you're setting up a shot with multiple lights in a structured way, it doesn't buy you much.

  • Your first paragraph is spot on, but unfortunately I can only accept a single answer... – kazanaki May 29 '16 at 15:13
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    Re: your last paragraph. If you are shooting a wedding reception or other event in a dimly lit banquet hall and have several off camera flashes placed around the room that you wish to balance with the ambient light to preserve the atmosphere of the party while also having properly exposed subjects the ability to control all of them with a simple adjustment of group ratios and FEC and letting E-TTL sort out the actual power levels is invaluable compared to having to go through a menu and manually set the power for each one each time you or your subject(s) move with respect to the lights. – Michael C May 30 '16 at 4:51
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    @Michael Fair enough; that's not something I ever do but I can see how it would be very valuable to those who do. Edited. – mattdm May 30 '16 at 16:34
  • In a studio situation, the only advantage I can think of is the ability to easily change your aperture without having to readjust your lighting. It has lots of advantages when used along with changing ambient light. If I recall correctly, Joe McNally always uses TTL, and he seems to know what he is doing ;) – Robin May 30 '16 at 17:12

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